410 EANDOLPH WEST 



through the vitelline vessels three were injected directly into 

 the posterior lymph heart plexus and one (12 mm.) was not in- 

 jected at all. All material was fixed in Zenker's fluid. Thirty- 

 six of the embryos were cut into 10 yu and 7 m serial sections and 

 stained on the slide with eosin and methyl blue by Mann's method. 

 One or two series were stained with Delafield's hemotoxylin 

 and orange G, but this method gave a very poor differentiation 

 of the blood cells. The nine embryos not sectioned were cleared 

 by the Spateholz method and examined in tola imder the binocu- 

 lar microscope (table 1). 



OBSERVATIONS 



A. FORMATION OF BLOOD CELLS FROM THE MESENCHYME AND 

 THEIR ENTRANCE INTO THE CIRCULATION VIA THE DEVEL- 

 OPING HAEMAL CAPILLARIES, PRIOR TO THE FORMATION OF 

 LYMPHATICS 



As the appearance of numerous })lood cells in the mesenchyme 

 and the extension of the haemal capillaries, previously referred 

 to, is the first change which occurs in the mesenchyme lateral 

 to the caudal muscle plate in the caudal region of the embryo, 

 these processes will be considered first. When the lymphatic 

 anlagen first appear, in the 10.5 mm. embryo, the haemal capil- 

 lary plexus has reached a very high degree of complexity and 

 from this time onward merely holds its own or develops compara- 

 tively slowly. 



The youngest embryo examined was one of 6.75 mm. In 

 this specimen the mesenchyme lateral to the muscle plate was 

 uniformly loose, and very nearly indifferent. A few rather 

 rounded eosinophile cells were observed in each section. Some of 

 these cells contained one or two large eosinophile granules. Oc- 

 casional venous branches pierced the muscle plate to drain the 

 mesenchyme lateral to it. 



The same area in the 7 mm. embryo presents several changes. 

 The mesenchyme is much more compact, being equal in den- 

 sity to the mesenchyme which lies medial to the muscle plate. 

 Groups of differentiating blood cells are much more abundant. 



